Current x-ray imaging systems may employ an x-ray tube providing a beam of x-rays emanating from a focal spot of the x-ray tube. The x-rays may be received by an image intensifier producing a visible image recorded by a video camera or the like. An object to be imaged is placed within the cone beam of x-rays and the video camera records an image indicating the attenuation of the x-ray beam by the imaged object.
An x-ray tube provides an electrical cathode within an evacuated envelope. Electrons generated at the cathode are accelerated against a target anode to produce x-rays. Controlling the electrical current generally affects the number of x-ray photons per unit time or fluence of the x-ray beam. Controlling the voltage between the cathode and anode affects the energy of each photon or the "hardness" of the x-rays.
In producing an x-ray image, it is often desirable to limit the dose to the extent possible. At the same time, it is desired that the imaging technique, i.e. the voltage across the tube and the current provided to the x-ray tube, be properly adjusted to provide an image with adequate detail. Generally, this adjustment considers the contrast in the image and its signal-to-noise ratio.
The correct technique varies considerably depending on the object being imaged. For medical imaging, it is known to provide certain preset techniques for different body parts. However, use of these presets requires the operator to identify the body part being imaged and will typically be less than optimum as a result of variations in particular patients and even the particular portion of the body part being imaged.
For many imaging situations where real-time imaging is required, it would be desirable to be able to turn the x-ray tube on and off on demand to obtain an instantaneous image and then to stop additional doses. The time required to adjust the proper technique for the particular imaged object is a significant obstacle to this goal.
Automatic exposure control (AEC) of an x-ray tube by varying the current to the x-ray tube based on the flux received by the image intensifier is known. Such AEC systems often work poorly when the imaged object is smaller than the field of view (FOV) of the system and therefore where some unattenuated x-rays are received by the image intensifier. In such cases, the AEC tends to overly decrease the x-ray fluence producing an image of the object that is too dark.
It is known to lower the total dose needed to produce a fluoroscopic image through the use of an image intensifier which uses electrical fields to accelerate photon produced electrons against a phosphorescent target.
Such image intensifiers tend to distort the image. Such distortion detracts from the usefulness of the image when instruments are manipulated by an operator viewing the image, especially near the edges of the field of view. Distortion also adversely affects quantitative uses of the image such as morphometric or densiometric analysis.
X-ray imaging systems having movable x-ray tubes and image intensifiers may produce an image on a stationary monitor that appears to rotate depending on the orientation of the machine. Often the operator will desire that the rotational orientation of the image be corrected to provide more intuitive view of the object. This is particularly the case in medical systems where the x-ray image is used to guide medical instruments. Prior art has addressed this problem through the use of a motorized rotating camera or movable deflection yokes on the display screen itself. Both of these approaches provide real-time rotated images.